Activin receptor-like kinase 4 (ALK4) is a type I transforming growth factor-? (TGF-?) superfamily receptor that mediates signaling for several TGF-? superfamily ligands including activin, nodal and GDF1. Inactivating mutations in ALK4 occur in a small percentage of breast and pancreatic cancers. In addition, ALK4 expression is frequently decreased/lost in breast and pancreatic cancer patients, with loss associated with a poor prognosis. ALK4 was also identified as a gene whose disruption cooperates with oncogenic Ras to promote pancreatic cancer progression in a murine model. While these data suggest an important role for loss of ALK4 function in breast and pancreatic cancer progression, the mechanism by which ALK4 contributes to cancer progression is unknown. We have demonstrated that loss of ALK4 expression coincides with metastatic potential in breast and pancreatic models, with silencing of ALK4 expression increasing cancer cell migration and invasion, markers of epithelial-mesenchymal transition, and canonical TGF-? signaling. Mechanistically, silencing ALK4 expression increases type I (T?RI/ALK5) and type II (T?RII) TGF-? receptor levels. Based on these preliminary studies, we hypothesize that loss of ALK4 promotes breast and pancreatic cancer invasion and metastasis by decreasing TGF-? receptor internalization, increasing TGF-? receptor levels, canonical TGF-? signaling and TGF-?-induced EMT. This hypothesis will be addressed by 2 aims. Specific Aim 1: To determine whether loss of ALK4 promotes TGF-? signaling and EMT by decreasing TGF-? receptor internalization and increasing TGF-? receptor levels. In this aim, we will (a) determine whether ALK4 inhibits TGF-? ligand binding, receptor complex formation, stability and/or internalization of TGF-? receptors; (b) determine whether Smad (2/3, 1/5/8) and/or non-Smad (MAPK, PI3K, NF-kB) signaling pathways are promoted by loss of ALK4; (c) identify TGF-? target genes induced by ALK4 silencing in cancer cell lines and human cancer specimens and (d) establish whether loss of ALK4 mediates EMT and invasion through these effects on TGF-? signaling. Specific Aim 2: To determine whether loss of ALK4 promotes cancer metastasis in vivo. In this aim, we will (a) determine whether restoring ALK4 expression in metastatic breast and pancreatic cancer cells decreases their metastatic potential in vivo, and whether decreasing ALK4 expression increases invasiveness and metastatic potential of breast and pancreatic cancer cells in vivo; (b) determine whether ALK4 loss induces cancer metastasis via increasing TGF-? receptor expression; and (c) investigate whether metastasis derived from cells with decreased ALK4 expression are sensitive to clinically developed TGF-? receptor inhibitors. These studies will define mechanisms by which ALK4 loss promotes cancer progression, establish a novel mechanism for regulating TGF-? signaling and potentially identify ALK4 loss as a predictive biomarker for anti-TGF-? approaches.